Poly (2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) is useful as a molding resin, possesses excellent weatherability for outdoor applications, and possesses low birefringence needed for compact discs. Polycarbonates are most commonly made by using phosgene. However, use of the highly toxic phosgene and organic solvents, as well as the expense of solution polymerization methods, has prompted chemists to find other methods of producing polycarbonates.
Melt preparation of poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) is challenging because it has a melting point much higher than its decomposition temperature. At decomposition temperatures, poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) ring-opens to produce carbon dioxide and 2-methyl-4,4-dimethyl-2-pentenal.
U.S. Pat. No. 5,171,830 discloses a melt method for making polyalkylene carbonates by reacting a glycol having at least 4 carbon atoms separating the hydroxyl groups with a diester of carbonic acid in the presence of a catalyst. This excludes 2,2,4,4-tetramethyl-1,3-cyclobutanediol, which has only 3 carbons separating the hydroxyl groups. Glycols which possess hydroxyl groups that are separated by 2 or 3 carbon atoms present a special problem for polycarbonate preparation by methods known in the art. These glycols tend to cyclize to produce five and six membered cyclic carbonates, which tend to vaporize.
Defensive Publication T873,016 discloses a method of producing low molecular weight poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) by self-condensing the diethyl ester of 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Defensive Publication T873,016 also discloses a method of reacting the glycol using diphenyl carbonate, which results in the formation of a colored polymer.
Methods for producing polycarbonates disclosed in the art produce low molecular weight polycarbonates when 2,2,4,4-tetramethyl-1,3-cyclobutanediol is employed as the glycol component. Japanese Patent 62-155370 discloses a process of reacting dialkyl carbonates with a glycol in the presence of a titanium catalyst. U.S. Pat. No. 3,022,272 discloses a process of reacting a carbonate, excluding dimethyl carbonate, with a glycol. Diphenyl carbonate is disclosed as the preferred carbonate, even though aromatic carbonates produce colored polymers. Lastly, U.S. Pat. No. 3,313,777 discloses a method of producing low molecular weight poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) by reacting dibutyl carbonate with the glycol. Moreover, the resultant polymer solidified in the reaction vessel, requiring very impractical recovery measures to be taken.
In previous polycarbonate processes, dimethyl carbonate has generally not been used in the preparation of polycarbonates, even though it is the least expensive and most readily available dialkyl carbonate known in the art. Dimethyl carbonate is the only dialkyl carbonate to form an azeotrope with its alkanol.
In light of the above, it would be desirable to have a process for preparing a polycarbonate from 2,2,4,4-tetramethyl-1,3-cyclobutanediol and dimethyl carbonate. None of the references discussed above disclose the efficient an method for preparing poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate).